FIG. 5 shows a direct sequence system which is one of spread spectrum communication systems.
In FIG. 5, a transmitter of the aforegoing system generally comprises a carrier wave generator 1, a mixer 2, a transmission pseudo noise code generator 3 and an antenna 6a. A receiver generally comprises a mixer 4, a reference pseudo noise code generator 5 and an antenna 6b.
A carrier wave outputted from the generator 1 (and modulated by information) and a pseudo noise code outputted from the pseudo noise code generator 3 are multiplied by the mixer 4, and a spread signal of a spectrum obtained from the mixer 4 is emitted from the antenna 6a. The receiver originally prepares in the reference pseudo noise code generator 5 a reference pseudo noise code which is identical to a pseudo noise code included in a received signal and has a phase identical to same, and multiplies it with a signal received by the antenna 6b in the mixer 4 to demodulate the carrier wave first modulated by information.
In a communication system as shown in FIG. 5, immediate and proper phase synchronization of the reference pseudo noise code of the receiver is one of the most important technologies. The phase synchronization process is divided into two steps, i.e. an initial synchronization process for finding a synchronous phase and a process for tracking the detected phase. Known technologies for the initial synchronization are sliding correlators, matched filters and others. Known tracking technologies are a tau-dither tracking and a delay lock tracking. Such a sliding correlator is configured to continuously change the phase of a reference pseudo noise code by slightly shifting the frequency of a reference pseudo noise code generating clock with respect to a transmission pseudo noise code generating clock in order to find a synchronous phase. Instead of shifting the frequency, the code phase may be varied stepwisely. However, both cases require a long time until the synchronous phase is detected. The use of a matched filter is effective to decrease the time.
Such a matched filter is a transversal filter weighted by a predetermined code pattern. One period or part of one period of a transmission pseudo noise code pattern is used as the code pattern, synchronization detection is greatly speeded up.
FIG. 6 shows an example of a delay line matched filter in which adders 8 and 9 are connected to taps of a delay line 7, and a phase inverter 10 is connected to the adder 9, so that an adder 11 adds outputs of the inverter 10 and adder 8.
When synchronization detection is performed by the matched filter, it is followed by a tracking process of the detected phase. The tracking is performed, using a delay lock circuit as shown in FIG. 7.
In FIG. 7, an IF amplifier 13 and a demodulator 14 are connected in sequence to a mixer 12 in which an input signal is entered. Mixers 15 and 16 are connected in parallel with the mixer 12, and their output stages are connected to envelope detectors 17 and 18. Outputs of the envelope detectors 17 and 18 are connected to a differential amplifier 19. The differential amplifier 19 are connected in series to a low pass filter 20, clock generator 21 and a pseudo noise code generator 22. An output of the pseudo noise code generator 22 is applied to the mixers 15 and 16. A 1/2 clock delay circuit 23 is provided for the pseudo noise code generator 22 to apply its output clock to the mixer 12.
Operation of the circuit of FIG. 7 is explained below, referring to a time chart of FIG. 8.
A pseudo noise code and an input signal are multiplied by the mixers 15 and 16 under different pseudo noise codes. Their detection outputs A and B have a time difference T as shown in FIG. 8. The detection outputs A and B are entered in the amplifier 19, and an output waveform C of the amplifier 19 exhibits a form summing both inputs, with its intermediate point being a tracking point Q.
Details of the aforegoing circuit is described in "Spread Spectrum Systems" by R. C. Dixon.
A matched filter is effective in order to obtain a high speed initial synchronization. However, since the detectable pseudo noise code is fixed, a number of matched filters are required in case of a code division multiple access. In this connection, an attention is paid in recent years to a convolver which can be used as a programmable matched filter. One of such convolvers is a monolithic ZnO/Si Sezawa wave convolver reported under the title of "Efficient Monolithic ZnO/Si Sezawa Wave Convolver" in Ultrasonics Symposium of IEEE 1982.
FIG. 9 is a block diagram showing an example of a correlator.
A carrier wave generator produces a carrier wave having a frequency fo. A reference pseudo noise code generator 25 produces a reference pseudo noise code. Their outputs are applied to a mixer 27. An output of the mixer 27 is applied to a convolver 28 together with the received signal fo, and a signal 2fo is outuputted. The reference pseudo noise code is time-inverted with respect to a pseudo noise code of the transmitter side, and the center frequency of its output is twice the input. The time width of its correlation output is set at a half clock period with respect to the clock period in the system using a matched filter.
The use of the convolver 28 in lieu of a matched filter for synchronization detection of the initial synchronization provides a spread spectrum receiver using a desired pseudo noise code.
As to synchronization detection, D. Brod & Korb proposed "Fast Synchronization in a Spread-Spectrum System based on Acoustoelectric Convolvers" in Ultrasonics Symposium of 1978 IEEE. The system includes a counter in a receiver to determine how much the phase of the reference pseudo noise code should be shifted in the initial synchronization. This is explained below, referring to FIG. 11.
Td designates the delay time of a convolver 30, A denotes a received pseudo noise code, and A is a reference pseudo noise code obtained by time-inverting the pseudo noise code A. Code A and code A advance in opposite directions in the convolver 30. When the head of code A is entered in the convolver 30, the counter starts its operation. When the counter counts t1 seconds, both codes coincide, and a large correlation output is obtained. By detecting this, it is recognized that both codes coincide at t1 seconds after the counting is commenced. It takes another t1 seconds for the head of the code A to reach the convolver input. Therefore, by measuring time t1 in the counter, the phase difference between both codes is known. It should be noted that one period length T.sub.PN of the pseudo noise code is shown in FIG. 11 as being equal to the delay time Td.
By employing the aforegoing method for establishing initial synchronization, high-speed initial synchronization is expected.
A means including a delay lock for tracking the detected phase difference is explained below, referring to FIG. 12.
One input end of a convolver 31 is connected in series to a mixer 32, a pseudo noise code generator 33 and a clock generator 34. The other input end of the convolver 31 is supplied with a received signal. A carrier wave generator 35 is connected to the mixer 32 to superpose a carrier on a pseudo noise code. On the other hand, the output end of the convolver 31 is connected in series to an envelope detector 36, a threshold detector 37 and a code phase adjuster 38. The code phase adjuster 38 is connected to a pseudo noise code generator 39 which is supplied with an output from the clock generator 35 and a start signal input. Further, a delay lock circuit 40 is provided for producing an LPF output and information output to the clock generator 34, based on the received signal input and an output of the pseudo noise code generator 39, respectively.
In FIG. 12, an output 2fo of the convolver 31 is entered in the code phase adjuster 38 after demodulation and threshold detection. The code phase adjuster 38 is configured to measure time t1 shown in FIG. 11 and adjust the code phase of the pseudo noise generator 39. The pseudo noise generator 39 controls the delay lock circuit 40 and tracks the above-mentioned phase difference.
In the described receiving system, high accuracy is required in the delay time Td of the convolver. For example, as shown in FIG. 13, there is a case where Td is larger by .DELTA.t than T.sub.PN. In FIG. 13, the pseudo noise code of the pseudo noise code generator 39 of FIG. 12 is shown by A'. When t=t1, a correlation output is obtained. It is at the time of t=2t1-.DELTA.t and not t=2t1 shown in FIG. 11 that the head of the received pseudo noise code A is entered in the convolver 31 subsequently. If .DELTA.t is already known, the pseudo noise generator 39 may be adjusted by the code phase adjuster 38 so that code A' starts from its head when t=2t1-.DELTA.t.
In the prior art spread spectrum receiver, however, if the .DELTA.t is not known, a phase error corresponding to .DELTA.t is produced between the pseudo noise codes A and A'. For example, when Td=9 .mu.sec and clock cycle T=0.1 .mu.sec, Td error amounts 9.+-.0.05 .mu.sec because the error in the initial synchronization must be .+-.T/2 as will be understood from FIG. 8. Therefore, there is a possibility of exceeding the variety of the convolver as manufactured. The variety can be eliminated by selecting products having acceptable Td. However, this apparently increases the manufacturing cost.
Additionally, there is possibility that Td exceeds an acceptable range due to the temperature characteristic of the convolver, and this prevents a reliable reception.